1. Field of the Invention
This invention relates to cardiac pacemakers in general and more specifically to physiologically responsive rate adaptive pacemakers. In particular, the invention is directed to a rate adaptive pacemaker responsive to variations in the cardiac A-V interval.
2. Description of Related Art
Cardiac pacemakers that provide pacing stimuli to the heart at a predetermined rate are well known in the art. Such pacemakers have taken a variety of forms including fixed rate, demand, and atrial or ventricular synchronous pacers.
Such pacemakers, while in many cases providing sufficient cardiac stimulation to sustain life, are incapable of responding to the increased cardiac demand that accompanies physical exertion. As a result, patients wearing such pacemakers have been severely limited with respect to the physical activities they may engage in.
In the past, a variety of rate adaptive pacemakers have been proposed to address this problem. Of particular interest in the present case are those that are intended to respond to various physiological parameters such as the oxygen content of the blood (U.S. Pat. No. 4,399,820 to Wirtzfield), blood pressure (U.S. Pat. No. 3,828,371 to Purdy; U.S. Pat. No. 3,638,656 to Grandjean), respiratory volume (U.S. Pat. No. 3,593,718 to Krasner), and blood pH (U.S. Pat. No. 4,009,721 to Alcidi). While such physiologically responsive rate adaptive systems for the most part comprise an improvement over the known fixed rate pacemakers, at least insofar as adaptability is concerned, they also generally exhibit a number of deficiencies.
For example, the foregoing known physiologically responsive rate adaptive systems require the use of physiological sensors in addition to the pacemaker unit. The requirement of an additional sensor increases both the expense and the risk of component failure in such systems. Moreover, it has been found difficult in many cases to design sensors that can accurately measure the parameters of interest and that are sufficiently reliable to be suitable for long term implantation. It has also been found that some of the proposed physiological parameters may vary in response to artificial stimuli such as certain medications, as well as to naturally occurring stimuli, thus presenting the risk that inappropriate and undesired variations in the pacing rate may occasionally occur.
Studies have shown that another type of physiological parameter, namely the cardiac Q-T interval, also varies with physical exertion and that this variation can be detected and used to adapt the pacing rate accordingly. See, for example, the following publications: Richards, Akhras, and Baron, Effects of Heart Rate on QT Interval, Proceedings of the VI World Symposium on Cardiac Pacing 2:7 (Meere ed. 1979); Rickards and Norman, Relationship Between QT Interval and Heart Rate: New Design of Physiologically Adaptive Pacemaker, British Heart Journal 45:56-61 (1981); Donaldson, Fox, and Rickards, Initial Experience With A Physiological, Rate Responsive Pacemaker, British Medical Journal 286:667-671 (February, 1983). U.S. Pat. No. 4,228,803 to Rickards discloses a rate adaptive pacemaker based on the above concept that is responsive to the interval between the delivery of a pacing stimulus and the T-wave evoked thereby, hereafter referred to as the S-T interval. In the Rickard's '803 pacemaker, a pacing stimulus generator activates a T-wave detector and a S-T interval timer when a pacing stimulus is generated. The S-T interval timer times the interval between the delivery of the pacing stimulus and the detection of the evoked T-wave by a T-wave detector. The timed interval is compared with the previously timed S-T interval and the difference is used to modify the pacer escape interval.
The use of the Q-T or S-T interval as the indicator of increased cardiac demand for controlling pacing rate has advantages over the foregoing proposed physiologically responsive, rate adaptive systems. First, a pacemaker utilizing the Q-T or S-T interval as the pacing rate control parameter does not require an additional sensor external to the pacemaker unit. Thus, both cost and risk of component failure are reduced. Second, the foregoing studies have found that the Q-T interval varies primarily with the increased circulation of catecholamines in the blood stream that results directly from physical exertion and that the Q-T interval is more insensitive to variations in heart rate due to other stimuli such as medications than some other proposed parameters.
Rate adaptive pacemakers responsive to the Q-T and S-T intervals also possess several inherent disadvantages, however. As the foregoing studies have found, the T-wave is often difficult to detect due to its relatively small magnitude (typically about 2 mv) and typically extended profile. In addition, the shape of the T-wave tends to vary with postural changes making accurate detection thereof even more difficult. Further, the studies have shown the Q-T interval may also tend to decrease somewhat with increases in heart rate caused by stimuli other than physical exertion. Therefore, there is a risk that a pacemaker that is responsive to the Q-T interval may induce tachycardia under certain conditions.
The present invention overcomes the inherent disadvantages of the known Q-T and S-T responsive pacemakers while providing rate adaptive pacing completely responsive to the physiology of the heart. The improvements of the invention are the result of the discovery that the A-V interval, i.e. the interval between the depolarization of the atrium and the depolarization of the ventricle, also varies directly with increased catecholamine circulation indicative of increased cardiac demand due to physical exertion. Use of the A-V interval as the pacing rate control parameter obviates the need to detect the T-wave. Instead, the P-wave which accompanies atrial depolarization and the Q-wave which accompanies the subsequent ventricular depolarization are detected. Both waves typically are much more sharply defined than the T-wave. Thus, they are typically more accurately detected.
In addition, the A-V interval is a direct function of the conduction time through the A-V node. The A-V node exhibits a natural fatigue property characterized by increased conduction time in response to increased heart rate unaccompanied by increased catecholamine circulation. Therefore, the risk of pacemaker induced tachycardia present in Q-T responsive pacemakers is eliminated. In fact, a pacemaker embodying this invention and utilizing reduction in A-V interval as the pacing rate control parameter naturally tends to oppose increases in pacing rate unless accompanied by increased physical exertion.
In view of the foregoing, it is an object of the invention to provide an improved rate adaptive pacemaker that is responsive to the physiology of the heart and that does not require additional sensors to provide rate responsiveness.
It is another object of the invention to provide a pacemaker having additional features not found in the foregoing rate adaptive pacemakers including: (1) separate rate response functions for spontaneously occurring cardiac activity and for induced cardiac activity; (2) gradual rate response similar to that of a normally functioning human heart; and (3) a built-in bias toward spontaneous rather than paced cardiac activity.